The present invention relates to oscillator circuits and, in particular, to an oscillator circuit having an expanded operating range.
An oscillator circuit consists of at least two parts: 1) an amplifying portion (e.g., a transistor) that will produce power gain; and 2) a feedback portion that determines the operating frequency. To control the gain and establish the operating point, an impedance (e.g., a resistor) may be connected in series with the amplifying portion. The addition of such impedance causes an increase in the start-up time required for the oscillator to reach a stable rate of oscillation. The impedance decreases the gain of the amplifying portion and increases the minimum supply voltage needed to initiate oscillations.
To ensure oscillator initiation, prior art oscillator circuits have taught that the supply voltage and/or loop gain may be momentarily increased during start-up. It has been learned that once oscillations are initiated, the supply voltage and/or loop gain may be reduced while oscillations are sustained. However, continuous operation at a high loop gain creates energy radiation or electromagnetic interference (EMI) as well as distortion while continuous operation at a high supply voltage increases the power dissipation of the oscillator.
To add further complication, oscillator initiation is also affected by supply voltage variation or, in other words, fluctuation in the power supplied to the oscillator. For example, in automotive electrical architectures the power supplied by the battery may vary the supply voltage between 9 volts and 16 volts. The operation of prior art oscillator circuits is affected by supply voltage variation. Accordingly, there is a desire to provide an oscillator circuit which overcomes the shortcomings of the prior art.